1. Field of the Invention
This invention relates to xerography and more particularly a xerographic method for preparing a master which can be used as a printing plate in either a gravure or lithographic mode or as a master to produce additional copies by xerographic techniques.
2. Description of the Prior Art
Lithographic printing is a well-known and established art. In general, the process involves printing from a flat plate, depending upon different properties of the image and non-image areas for printability. In conventional lithography the non-image area is hydrophilic while the image area is hydrophobic. In the lithographic printing process, a conversion solution is applied to the plate surface which wets all portions of the surface not covered by the hydrophobic image. This solution keeps the plate moist and prevents it from scumming up. An oil based printing ink is applied to the image surface depositing the lithographic ink only on the image area, the hydrophilic non-image area repelling the ink. The ink image may then be transferred directly to a paper sheet or other receptive surface, but generally it is transferred to a rubber off-set blanket which in turn transfers the print to the final paper sheet. Hence, for each print made during a run, the lithographic plate is first dampened with an aqueous conversion solution and then inked with a lithographic ink and finally printed.
It has been known that lithographic plates can be made in a photoconductive system by utilizing the conventional developed xerographic plate as a lithographic printing plate. The general process of utilizing a xerographic plate in a lithographic system is described in U.S. Pat. Nos. 3,107,169 and 3,001,872. In these systems, usually a zinc oxide-type plate is charged by conventional means, exposed to the image to be reproduced, and developed with conventional xerographic toner. The toner used is generally hydrophobic in nature as is the background of the conventional inorganic binder-type xerographic plate. In order that the developed xerographic plate be useful as a lithographic master, a differential must be established between the toned image and the background of the plate. Since both are hydrophobic nature, it has heretofore been required that the background of the xerographic plate be treated, by the use of a conversion solution, to make it hydrophilic in nature. After the conversion of the background, the plate is then wetted with a nonaqueous or oil based ink. The toner accepts the ink and the hydrophilic background repels the ink.
While basically this system has been found useful for lithographic purposes, there are inherent disadvantages to its use. One disadvantage, for example, is the fact that it requires a conversion solution to convert the initially hydrophobic background to a hydrophilic state so it will no longer accept the oil based ink during the inking step. A further disadvantage to this system is the lack of re-use of the zinc oxide type xerographic plate, since after the plate is developed and the toner fused, the plate can not be used again.
Other attempts have been utilized to produce a lithographic plate by removing the photoconductive layer upon which the initial xerographic image is formed. The photoconductive layer can be removed in the non-image areas by contacting the developed plate with an adhesive sheet as described in U.S. Pat. No. 3,446,616 or the photoconductive layer can just be removed from underneath the developed image area as described in Defensive Publication No. T878022. A further possibility is to completely remove all of the photoconductive layer by an adhesive surface transfer as described in Defensive Publication No. T878022.
The disadvantage of these photoconductive particle transfer methods is that a re-usable photoconductive layer cannot be employed. Furthermore, the production of each lithographic plate consumes nonreusable photoconductive particles which results in additional expense.
It is therefore an object of this invention to provide a master to be produced by xerography which can be employed in a lithographic mode to overcome the above noted disadvantages.
It is a further object of this invention to provide a novel method for the preparation of a lithographic master plate.
Another object of this invention is to provide an imaging system employing a re-usable photoconductive member which can serve as a master to make multiple copies without re-exposure to the original.
A still further object of this invention is to provide a method of producing a master in a quick and efficient manner which can be used in printing such as gravure printing.
Another object is to provide a master capable of producing additional copies by printing such as lithography, gravure or by xerography and at the same time producing a permanent record of the image.
These and other objects are achieved by the present method.